irq_multilevel: explicitly initialize all members to avoid a warning

As this is a header, let's explicitly initialize all members of the
struct to avoid a "missing initializer for member" warning in case the
file is built with -Wmissing-field-initializers, or worse, that warning
set to be an error.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>